I have a Sears 600 furnace; the burner cycles normally, but the blower is constantly on. There is a summer / auto switch that will basically turn off the furnace when on summer; no cycling at all. Any Ideas? Thanks.

Might have a loose wire at your thermostat. Make sure all the thermostat wires are secure at the thermostat and where they connect at the unit. Hope this helped.

If the unit is equipped with a combination fan/limit control, it may be stuck in the "ON" position. If so, try gently rotating the dial counter-clockwise to see if the fan shuts off.